03- Final Prototype

The major requirements needing to be addressed in our final prototype design included the following:

-Determine the packaging/layout of all parts, since our prototype did not include the knife or whetstone

-A cam follower mechanism design to replace the pure linkage system

-Design the structures to carry the whetstone and knife, with sufficient rigidity

-Bearing mounting for all components (to match overall layout) with sufficient rigidity. 

Our final design has a slider carrying the whetstone, with the driving mechanism for the knife rotation located beneath it. The most complex structure is the link carrying the knife. It is pivoting on an angled axle located under the slider, and must rigidly support the knife above the axle, while maintaining clearance for the whetstone movement. Things packaged better with the whetstone and the knife closer to parallel than perpendicular, as we envisioned in earlier concepts. However, the knife is still at a 33 degree angle to the long axis of the whetstone, which is important to allow the knife to move smoothly across the stone. This angle and the length of the stroke were chosen so that the knife uses as much of the length of stone as possible.

Our whetstone carrier consisted first of a lasercut platform bolted via 3d printed adapters to the linear bearings. Two drop plates were glued to this platform to carry the cam profile. The floating platform holding the whetstone rides on press-fit brass bushings on 4 shafts, sitting on compression springs for the necessary vertical compliance.

Our cam follower is pictured above. The profile moves along with the whetstone slider. For the first part of the travel, while the straight edge of the knife is being sharpened, the profile is flat and there is no actuation of the knife rotation. For the second part of the travel, the rotation actuation linkage follows a ramped profile that sweeps the full angle of the rounded section of the knife edge. The four-bar carrying the knife is spring loaded against the ramp profile, however the spring tension in this prototype was insufficient.

The remainder of the knife rotation linkage is pictured above, with the actuating/cam following link on the right. Because the link carrying the knife rotates on a non-vertical axis (as in our first prototype), we need one link with ball joints at either end to accommodate the out-of-plane motion. To accomplish the angled axis of rotation, we used a 3d-printed bearing carrier insert in the knife link. Packaging requirements meant that the actual knife was fairly far above the grounding joint of the link, so the arms are reinforced by cross struts for stability. Although the interfaces of this link are at relatively unwieldy locations and angles relative to one another, it was still worth designing this part for laser-cuttability for light weight and speed of manufacturing. 

Final Prototype Issues

The most obvious thing missing from this prototype is the actual driving mechanism for the device. The intent was to have a motor-driven crank actuating the whetstone slider.

Past that, there are a couple glaring issues impeding the functionality of our device. Firstly, we found that the brass bushings for the vertical motion of the whetstone tend to bind up very easily with any misalignment. This could potentially be helped by using a closer fit between the shafts and bushings, but this solution would also introduce the problem of requiring a very tight tolerance on the relative locations of the four shafts. A cost-effective solution may be to support the stone with a spring-loaded four bar or scissor lift mechanism.

We also had issues with the ease of motion of the whetstone in the horizontal direction. The linear bearings we used here also seemed more sensitive than anticipated to angular misalignment. We are overconstraining the slider by having it ride on two separate shafts, so we could remove one shaft and have a sliding interface on the other side. Alternatively, we could again mount the whetstone on a four-bar or simply a long single link.

The last big issue is that the cam follower was not actually interfacing with the profile until the very end of the slider travel. This appeared to be due to clearance issues as well as manufacturing error, enabled by a lack of a locating feature for the knife link bearing carrier.

While it was not an insurmountable issue, this project did give us some practical insight into why cam follower mechanisms have some drawbacks. Maintaining contact between the cam and cam profile is a non-negligible issue, and it seems that our usage of this mechanism contributed to the overall slop and rough motion. If we had been able to make a pure linkage system work, it would have likely have helped smooth the motion of our mechanism. This is clearly not an application demanding high precision or load capability, so it is easy to imagine these issues becoming much more difficult and cost-intensive to solve in those situations.

Mechanism Video:

Overall Images